1. Field of the Invention
The present invention relates to an endoscope objective optical system having a magnification change function and capable of magnifying observation.
2. Description of the Related Art
In recent years, there has been an increasing demand for an optical system capable of magnifying observation for accurate diagnosis of lesions. Japanese Patent Application Laid-Open Publication Nos. 4-218012 and 11-316339 disclose optical systems capable of magnifying observation, each of which is constituted by three groups of lenses: positive, negative and positive lens groups successively disposed from the object side, and in which the second negative lens group is moved to change the focal length, in other words, perform magnification change and focusing.
In endoscopes, a large depth of field and freedom from degradation in image quality from near point to a far point contribute to ease of observation and ease of finding a lesion. The depth of field is expressed by the following expression:Δx∝P×Fno./f2 where Δx is the depth of field; P is the pixel pitch of an image pickup device; Fno. is the F-number of an objective optical system; and f is the focal length of the objective optical system. From this expression, it can be understood that the characteristics of the depth of field are as described below.
(a) The larger the pixel pitch of the image pickup device, the larger the depth of field.
(b) The larger the F-number of the objective optical system (the smaller the aperture), the larger the depth of field.
(c) The smaller the focal length of the objective optical system, the larger the depth of field.
Also, there is a demand for higher image quality of endoscopic images to improve the accuracy of diagnosis, and the adoption of new image pickup devices having larger numbers of pixels is being pursued.
The adoption of an image pickup device having a larger number of pixels entails a reduction in depth of field, because the image pickup device has a reduced pixel pitch. Also, prevention of degradation in image quality in the event of a diffraction phenomenon caused by an increase in F-number, i.e., a reduction in aperture of an aperture stop, requires determining the specifications of the optical system so that the F-number satisfies the following Rayleigh limit condition expression.Fno.<1.64×P/λwhere Fno. is the F-number of the objective optical system; P is the pixel pitch of the image pickup device; and λ is the wavelength.
From the above expression, it can be understood that the F number cannot be made sufficiently large if the pixel pitch P is reduced.
For the above-described reason, the depth of field of the objective optical system used for image pickup device is reduced due to reductions in pixel pitch and F number of image pickup devices with improvement in image quality of endoscopes.